The most important cooling lines of the neutral interstellar medium ( ISM ) lie in the far-infrared ( FIR ) .
We present measurements by the Infrared Space Observatory Long Wavelength Spectrometer of seven lines from neutral and ionized ISM of 60 normal , star-forming galaxies .
The galaxy sample spans a range in properties such as morphology , FIR colors ( indicating dust temperature ) , and FIR/Blue ratios ( indicating star-formation activity and optical depth ) .

In two-thirds of the galaxies in this sample , the [ \ion C2 ] line flux is proportional to FIR dust continuum .
The other one-third show a smooth decline in { L _ { [ CII ] } } / { L _ { FIR } } with increasing { { F _ { \nu } ( 60 \mu { m } ) } / { F _ { \nu } ( 100 \mu { m } ) } } and { L _ { FIR } } / { L _ { B } } , spanning a range of a factor of more than 50 .
Two galaxies , at the warm and active extreme of the range have { L _ { [ CII ] } } / { L _ { FIR } } < 2 \times 10 ^ { -4 } ( 3 \sigma upper limit ) .
This is due to increased positive grain charge in the warmer and more active galaxies , which leads to less efficient heating by photoelectrons from dust grains .

The ratio of the two principal photodissociation region ( PDR ) cooling lines { L _ { [ OI ] } } / { L _ { [ CII ] } } shows a tight correlation with { { F _ { \nu } ( 60 \mu { m } ) } / { F _ { \nu } ( 100 \mu { m } ) } } , indicating that both gas and dust temperatures increase together .
We derive a theoretical scaling between [ \ion N2 ] ( 122 \mu { m } ) and [ \ion C2 ] from ionized gas and use it to separate [ \ion C2 ] emission from neutral PDRs and ionized gas .
Comparison of PDR models of Kaufman et al .
( 1999 ) with observed ratios of ( a ) { L _ { [ OI ] } } / { L _ { [ CII ] } } and ( { L _ { [ CII ] } } + { L _ { [ OI ] } } ) / { L _ { FIR } } and ( b ) { L _ { [ OI ] } } / { L _ { FIR } } and { { F _ { \nu } ( 60 \mu { m } ) } / { F _ { \nu } ( 100 \mu { m } ) } } yields far-UV flux G _ { 0 } and gas density n .
The G _ { 0 } and n values estimated from the two methods agree to better than a factor of 2 and 1.5 respectively in more than half the sources .

The derived G _ { 0 } and n correlate with each other , and G _ { 0 } increases with n as G _ { 0 } \propto n ^ { \alpha } , where \alpha \approx 1.4 .
We interpret this correlation as arising from Strömgren sphere scalings if much of the line and continuum luminosity arises near star-forming regions .
The high values of PDR surface temperature ( 270 - 900 K ) and pressure ( 6 \times 10 ^ { 4 } -1.5 \times 10 ^ { 7 } K cm ^ { -3 } ) derived also support the view that a significant part of grain and gas heating in the galaxies occurs very close to star-forming regions .
The differences in G _ { 0 } and n from galaxy to galaxy may be due to differences in the physical properties of the star-forming clouds .
Galaxies with higher G _ { 0 } and n have larger and/or denser star-forming clouds .